feat: add w3c headers

[darklight3it]

Description of changes:

Add W3C distributed tracing support to aws-lambda-invoke-store. This introduces three new protected context fields and their corresponding read-only accessors:

• getTraceparent() — W3C Trace Context traceparent header
• getTracestate() — W3C Trace Context tracestate header
• getBaggage() — W3C Baggage header

These fields follow the same immutability contract as the existing REQUEST_ID and X_RAY_TRACE_ID keys: they are set once via run() and cannot be modified by user code afterwards.

Testing:

• Added unit tests covering getter behavior, undefined-when-absent, and write-protection for all three new fields (both single and multi-concurrency modes).

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kuhe]

this repo is using changesets, I thought? PR might need a changeset file

[trivikr]

The changeset bot was removed from the repo for some reason.
It did post message in March in https://www.github.com/awslabs/aws-lambda-invoke-store/pull/34#issuecomment-4038958816 for example.

I've enabled it as of now. I'll request rebase to check if it runs.

---

UPDATE: Changeset bot was run on rebase and posted comment below

[changeset-bot]

🦋 Changeset detected

Latest commit: a7c6e9d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@aws/lambda-invoke-store	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[trivikr]

This needs a changeset file. This should be a minor bump since new APIs are being added.

[darklight3it]

@trivikr

This needs a changeset file. This should be a minor bump since new APIs are being added.

Added the changeset and some more tests.